Osteoarthritis (OA) is a chronic disease in which the articular cartilage that lies on the ends of bones that forms the articulating surface of the joints gradually degenerates over time. There are many factors that are believed to predispose a patient to osteoarthritis including genetic susceptibility, obesity, accidental or athletic trauma, surgery, drugs and heavy physical demands. Osteoarthritis is initiated by damage to the cartilage of joints. The two most common injuries to joints are sports-related injuries and long term “repetitive use” joint injuries. Joints most commonly affected by osteoarthritis are the knees, hips and hands. In most cases, due to the essential weight-bearing function of the knees and hips, osteoarthritis in these joints causes much more disability than osteoarthritis of the hands. As cartilage degeneration progresses, secondary changes occur in other tissues in and around joints including bone, muscle, ligaments, menisci and synovium. The net effect of the primary failure of cartilage tissue and secondary damage to other tissues is that the patient experiences pain, swelling, weakness and loss of functional ability in the afflicted joint(s). These symptoms frequently progress to the point that they have a significant impact in terms of lost productivity and or quality of life consequences for the patient.
Articular cartilage is predominantly composed of chondrocytes, type II collagen, proteoglycans and water. Articular cartilage has no blood or nerve supply and chondrocytes are the only type of cell in this tissue. Chondrocytes are responsible for manufacturing the type II collagen and proteoglycans that form the cartilage matrix. This matrix in turn has physical-chemical properties that allow for saturation of the matrix with water. The net effect of this structural-functional relationship is that articular cartilage has exceptional wear characteristics and allows for almost frictionless movement between the articulating cartilage surfaces. In the absence of osteoarthritis, articular cartilage often provides a lifetime of pain-free weight bearing and unrestricted joint motion even under demanding physical conditions.
During fetal development, articular cartilage is initially derived from the interzone of mesenchymal condensations. The mesenchymal cells cluster together and synthesize matrix proteins. The tissue is recognized as cartilage when the accumulation of matrix separates the cells, which are spherical in shape and are now called chondrocytes. During cartilage formation and growth, chondrocytes proliferate rapidly and synthesize large volumes of matrix. Prior to skeletal maturity, chondrocytes are at their highest level of metabolic activity. As skeletal maturation is reached, the rate of chondrocyte metabolic activity and cell division declines. After completion of skeletal growth, most chondrocytes do not divide but do continue to synthesize matrix proteins such as collagens, proteoglycans and other noncollagenous proteins (1, 2).
Like all living tissues, articular cartilage is continually undergoing a process of renewal in which “old” cells and matrix components are being removed (catabolic activity) and “new” cells and molecules are being produced (anabolic activity). Relative to most tissues, the rate of anabolic/catabolic turnover in articular cartilage is low. Long-term maintenance of the structural integrity of mature cartilage relies on the proper balance between matrix synthesis and degradation. Chondrocytes maintain matrix equilibrium by responding to chemical and mechanical stimuli from their environment. Appropriate and effective chondrocyte responses to these stimuli are essential for cartilage homeostasis. Disruption of homeostasis through either inadequate anabolic activity or excessive catabolic activity can result in cartilage degradation and osteoarthritis (3). Most tissues that are damaged and have increased catabolic activity are able to mount an increased anabolic response that allows for tissue healing. Unfortunately, chondrocytes have very limited ability to up-regulate their anabolic activity and increase the synthesis of proteoglycan and type II collagen in response to damage or loss of cartilage matrix. This fundamental limitation of chondrocytes is the core problem that has precluded the development of therapies that can prevent and cure osteoarthritis. Additionally, there is a need for a definitive diagnostic test for detecting early osteoarthritis, and a prognostic test that effectively monitors a patient's response to therapy.
Joint pain is the most common manifestation of early osteoarthritis. The pain tends to be episodic lasting days to weeks and remitting spontaneously. Although redness and swelling of joints is uncommon, joints become tender during a flare-up of osteoarthritis.
“Mild” or “early stage osteoarthritis” is difficult to diagnose. The physician relies primarily on the patient's history and physical exam to make the diagnosis of mild osteoarthritis. X-rays do not show the underlying early changes in articular cartilage. There are no recognized biochemical markers used to confirm the diagnosis of early stage osteoarthritis.
X-ray changes confirm the diagnosis of moderate osteoarthritis. X-rays of normal joints reveal well preserved symmetrical joint spaces. Changes seen on the x-rays of patients with osteoarthritis include new bone formation (osteophytes), joint space narrowing and sclerosis (bone thickening). There are no recognized biochemical markers used to confirm the diagnosis of “moderate osteoarthritis” at this stage.
The clinical exam of a joint with severe osteoarthritis reveals tenderness, joint deformity and a loss of mobility. Passive joint movement during examination may elicit crepitus or the grinding of bone-on-bone as the joint moves. X-ray changes are often profound: the joint space may be obliterated and misalignment of the joint can be seen. New bone formation (osteophytes) is prominent. Again, there are no recognized biochemical markers used to confirm the diagnosis of “severe osteoarthritis”.
“Osteoarthritis” is the most common chronic joint disease. It is characterized by progressive degeneration and eventual loss of cartilage. Currently, there is a need for an effective therapy that will alter the course of osteoarthritis. Further advances in preventing, modifying or curing the osteoarthritic disease process critically depends, at least in part, on a thorough understanding of the molecular mechanisms underlying anabolic and catabolic processes in cartilage. Since cellular functions are substantially determined by the genes that the cells express, elucidating the genes expressed in articular cartilage at different developmental and disease stages will inevitably provide new insights into the molecules and mechanisms involved in cartilage formation, injury, disease and repair.
cDNA libraries from putatively normal and severe osteoarthritic human cartilage tissue have been constructed (Kumar et al., 46th Annual Meeting, Orthopaedic Res. Soc., Abstract, p. 1031). However, this work does not adequately address the differentiation of chondrocyte gene expression from differing severities of osteoarthritic human cartilage (mild, moderate, marked and severe). In addition, the “normal cartilage” samples were obtained from deceased donors more than 24 hours after death. Thus, this cDNA library does not truly reflect normal chondrocyte gene expression due to the rapid degeneration of RNA that occurs after cessation of perfusion to the sampled joint, as demonstrated by baboon studies, presented herein below.